Levulinic acid is a well-known product of hexose acid hydrolysis 1,2!, it being formed in a 72/28 ratio with formic acid. Numerous attempts have been made of the past century to commercially produce levulinic acid, but its utilization on a large scale has never been accomplished. All current production is apparently limited to small operations in Europe and Japanese production was reported as recently as 1975. Developers in Florida 3! in the 1980s used levulinic acid derivatives, both 2-methyltetrahydrofuran (MTHF) and angelicalactone, as fuel blending agents, which is supported by single engine test stand data. Accordingly, levulinic acid conversion to MTHF is useful as a fuel or fuel additive. In addition, it may be useful for making polymer fibers.
Because levulinic acid may be inexpensively obtained from cellulose, for example pulp waste and/or agricultural waste, there is motivation to find an economic way of converting the levulinic acid to MTHF.
Direct production of MTHF from levulinic acid is not reported in the literature except as a minor byproduct 4!. Copper-chromium oxide was used to catalyze the hydrogenation in a two stage manner at 245.degree. C. and 300.degree. C. and required about 80 minutes. The reaction yielded 11% gamma-valerolactone and 44% 1,4-pentanediol with a 22% yield of a water byproduct with the "odor of a alpha-methyl tetrahydrofuran", but no quantitative analysis of the water byproduct was reported, (estimated 4.5 mol % yield of MTHF in the water byproduct).
However, there has been extensive reporting of processing of levulinic acid and its derivatives which eventually leads to MTHF production. There are two pathways of interest, first beginning with a dehydration step via angelicalactone, and the second beginning with hydrogenation of the levulinic acid to the 4-hydroxypentanoic acid (HPA)(aka hydroxyvaleric acid). The pathway through angelicalactone was proposed by the Floridians 3!, but no actual production of MTHF was reported. The hydrogenation path proceeding through HPA has been tested and the results reported in many publications with various final products indicated.
The dehydration of levulinic acid to angelicalactone is easily accomplished by simple heating of levulinic acid to about 160.degree. C., acid promotes the dehydration, and distillation of the products at atmospheric or reduced pressure of about 10 to 50 mm Hg 3,5,6! (FIG. 1). Angelicalactone and water products separate in the collector. The resulting angelicalactone can be catalytically hydrogenated to 1,4-pentanediol (PDO) using barium-stabilized copper-chromite catalyst at 240.degree. C. 6!. The PDO is readily converted to the MTHF by a dehydration reaction 7! (FIG. 2) owing to the thermal instability of the PDO. The acid-promoted reaction is accomplished in a similar manner to the formation of the angelicalactone from levulinic acid, i.e., by heating of the PDO in the presence of acid and the simple distillation of the MTHF product at 76.degree.-80.degree. C. A more recent version is the processing with Nafion.RTM.-H (Tradename of EI DuPont, Wilmington, Del.) resin in which a 90% yield was obtained within 5 hr at 135.degree. C. 8!.
Catalytic hydrogenation of levulinic acid has been reported to produce a number of different products dependent upon the catalyst and conditions. These differences result from the variations in processing severity and the resulting extent of progress down a sequential pathway shown in FIG. 3. Levulinic acid is reduced to HPA which readily dehydrates to .gamma.-valerolactone (GVL). GVL is hydrogenated to 1,4-pentanediol (PDO) which dehydrates to MTHF. Side products include pentanoic (aka valeric) acid and 1-pentanol.
Early published results claim the recovery of HPA after reduction of levulinic acid with the catalyst sodium amalgam, nickel catalyzed hydrogenation in the vapor phase at 250.degree. C. and electrocatalytically from an alkaline solution 10!. More recent studies claim the formation of the HPA product by hydrogenation using sodium metal in NaOH-ethanol solution at ambient conditions (60% yield after 4 hr) 10!, or Raney nickel in aqueous alkali at 75.degree. C. up to 250.degree. C. with 2500 psig over pressure of hydrogen (84.1% yield after 27 min) 11!, but in both cases the GVL product was recovered after dehydration of the HPA. A more recent study with Raney nickel at 10 to 90.degree. C. in aqueous alkali without added hydrogen 12! showed a combined product of HPA and GVL at conversions of 53 to 65% after 1 hour.
Catalytic production of the GVL from levulinic acid has been investigated by several groups. Schutte and Thomas 13! developed an hydrogenation process using platinum oxide catalyst in an organic solution of the levulinic acid. They were concerned with decomposition of the GVL at higher temperatures (above 160.degree. C.) 14! and questioned earlier reports of high temperature production 16!. Their studies showed a solvent effect on the room temperature reduction at 2-3 atm hydrogen over-pressure. A yield of 87% was achieved in ethyl ether after 44 hr of reaction. Reaction in acetic acid or ethanol proceeded more slowly with only 48% and 52% yield, respectively, after the 44 hr. These results compliment the work of Jacobs and Scott 15! who found that GVL was unreactive over platinum oxide in ethanol solution. Similar tests showed that AL was hydrogenated to GVL in this system.
A process for hydrogenation of levulinic acid was later patented wherein the catalysis is done in the neat liquid phase with a nickel catalyst at 900 psig hydrogen and 175.degree.-200.degree. C. 16!. The basis for the patent is found in a later article 4! where the results are given as 94% yield of GVL after 3 hr at 100.degree. C. up to 220.degree. C. and an initial hydrogen pressure of 700 psig with Raney nickel. Also reported in that article is the use of copper-chromite catalyst to produce a complex product of 11% GVL, 44% PDO and 22% of water solution possibly containing MTHF. The process was completed over a temperature range of 190.degree. C. up to 300.degree. C. over 80 min of reaction time. The reaction was performed in neat liquid phase at 267 atm pressure (200 atm hydrogen initial pressure). This is the first report of the MTHF byproduct, although its ready formation from PDO by thermal decomposition/dehydration 4! explains its presence. Christian et al. contrast their result with the earlier report 17! of direct reduction of the ketone functional group in GVL and other lactones to produce MTHF and related furans. Also reported is a second process using Raney nickel catalyst at less severe conditions (100 atm initial hydrogen pressure and 273.degree. C. reaction temperature) which produced 62% GVL and 21% PDO.
Two later papers describe rhenium catalysts for hydrogenation of levulinic acid to GVL. Rhenium black produced by in-situ reduction from rhenium heptoxide was used to produce a 71% yield of GVL from neat levulinic acid after 18 hr at 106.degree. C. and 150 atm pressure 18!. The balance of the product is described as polymeric esters. It is noted in the text that those hydrogenations run in anhydrous acids always resulted in some by-product ester formation; those run in water solvent gave markedly reduced ester formation, or in most cases no ester formation. In related experiments with acetic acid, the in-situ rhenium catalyst was found to be the only effective catalyst (at only 150.degree. C.) with platinum oxide, copper-chromite and nickel all inactive at temperatures of 250.degree.-300.degree. C. Another form of rhenium, Re(IV) oxide hydrate generated from ammonium perrhenate by reduction with zinc in H.sub.2 SO.sub.4, was also shown to have useful catalytic properties. Levulinic acid was converted 100% to GVL at 152.degree. C. and 200 atm pressure after 12 hr 19!.
Related literature of interest deals with the hydrogenation of GVL. Hydrogenation of GVL over copper-chromite catalyst was described at 250.degree. C. and 200-300 atm pressure 20!. The yield was 78.5% PDO and 8.1% 1-pentanol. In a later study 4! using a copper-chromite catalyst, up to 83% yield of PDO was achieved at 240.degree.-260.degree. C. In tests at higher temperature, 270.degree.-290.degree. C., PDO yields dropped to 32%, and an undisclosed amount of MTHF was found in a low-boiling product fraction.
More recent studies have focused on homogeneous catalysis of the hydrogenation steps. Joo et al. describe both ruthenium 21! and rhodium 22! complexes which can hydrogenate levulinic acid at low temperature (60.degree. C.) in aqueous solutions. However, no indication of products is given. A more interesting study has identified GVL as the product from levulinic acid when using ruthenium iodocarbonyl complexes 23!. These complexes converted 85-100% of the levulinic acid to GVL in 8 hr @ 150.degree. C. Other results with ruthenium triphenylphosphine complexes also show activity for the hydrogenation of levulinic acid to GVL, up to 99% conversion and 86% yield after 24 hr @ 180.degree. C. 24!, but these function in toluene solution and are not stable in water.
Although the literature describes useful processes for conversion of levulinic acid to MTHF, the processes require multiple steps with different catalysts and disparate operating conditions. Homogeneous catalysis has also been reported, but economic processing is not likely for the usual reasons of precious metal catalyst regeneration and recovery.
None of the prior art processes produced MTHF in a concentration or yield greater than about 4.5 mol %. Accordingly, there is a need for a fuel or fuel component of MTHF having a concentration greater than 4.5 mol %.
Further, there is a need in the art for a simpler conversion of levulinic acid to MTHF.